1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to novel semiconductor light-emitting devices with double-sided passivation that effectively reduces the leakage current and enhances the device reliability.
2. Related Art
Solid-state lighting is expected to bring the next wave of illumination technology. High-brightness light-emitting diodes (HB-LEDs) are emerging in an increasing number of applications, from serving as the light source for display devices to replacing light bulbs for conventional lighting. Typically, cost, efficiency, and brightness are the three foremost metrics for determining the commercial viability of LEDs.
An LED produces light from an active region which is “sandwiched” between a positively doped layer (p-type doped layer) and a negatively doped layer (n-type doped layer). When the LED is forward-biased, the carriers, which include holes from the p-type doped layer and electrons from the n-type doped layer, recombine in the active region. In direct band-gap materials, this recombination process releases energy in the form of photons, or light, whose wavelength corresponds to the band-gap energy of the material in the active region.
To ensure high efficiency of an LED, it is desirable to have the carriers recombine only in the active region instead of other places such as the lateral surface of the LED. However, due to the abrupt termination of the crystal structure at the lateral surface of the LED, there are large numbers of recombination centers on such surface. In addition, the surface of an LED is very sensitive to its surrounding environment, which may lead to added impurities and defects. Environmentally induced damage can severely degrade the reliability and stability of an LED. In order to insulate an LED from various environmental factors, such as humidity, ion impurity, external electrical field, heat, etc., and to maintain the functionality and stability of the LED, it is important to pay special care to maintain the surface cleanness and to ensure reliable LED packaging. Moreover, it is also critical to protect the surface of an LED using surface passivation, which typically involves depositing a thin layer of non-reactive material on the surface of the LED.
FIG. 1 illustrates a traditional passivation method for an LED with a vertical-electrode configuration. In FIG. 1, from top down, a layer 100 is the passivation layer, a layer 102 the n-side (or p-side) electrode, a layer 104 the n-type (or p-type) doped semiconductor layer, a layer 106 the active layer based on a multi-quantum-well (MQW) structure, a layer 108 the p-type (or n-type) doped semiconductor layer, a layer 110 the p-side (or n-side) electrode, and a layer 112 is the substrate.
The passivation layer blocks the undesired carrier recombination at the LED surface. For the vertical-electrode LED structure shown in FIG. 1, surface recombination tends to occur on the sidewalls of the MQW active region 106. However, the sidewall coverage by a conventional passivation layer, for example, layer 100 shown in FIG. 1, is often non-ideal. The poor sidewall coverage is typically a result of standard thin-film deposition techniques, such as plasma-enhanced chemical vapor deposition (PECVD) and magnetron sputtering deposition. The quality of sidewall coverage by passivation layer is worsened for devices with steeper steps, such as devices with steps higher than 2 μm, which is the case for most vertical-electrode LEDs. Under such conditions, the passivation layer often contains a large number of pores, which can severely degrade its ability to block the surface recombination of carriers. An increased surface recombination rate in turn increases the amount of the reverse leakage current, which results in reduced efficiency and stability of the LED.